When will that volcano stop erupting?

When Will That Volcano Stop Erupting?

It could be thousands of years.

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A state of emergency has been declared for part of Peru, where a dome of lava continues to expand from the crater of the Ubinas volcano. The volcano began to erupt three weeks ago, and over the weekend authorities told those remaining in local villages to evacuate. How long can an eruption last?

Hours, days, or years. The median length of a volcanic eruption is seven weeks. About half of recorded eruptions last less than two months, with a tenth taking no more than a day. Almost 20 percent go on for more than a year, and some last for much longer.

An eruption begins when volcanic gases or molten rocks start to pop out of the crater. You can get a pretty good idea of how long it will last by reviewing previous eruptions from the same volcano. Alaska's Augustine volcano started to erupt in January and has continued to belch lava on a time scale similar to that of the eruptions in 1976 and 1986. In all three events, an initial series of explosions took place over a week or two, followed by several months of dome-building eruption.

Every once in a while an eruption goes on much longer than expected. Kilauea volcano in Hawaii has been producing lava flows since 1983 without a break, despite a history of much shorter eruptions. The volcano on the Mediterranean island of Stromboli has been spitting out molten rock for at least 2,400 years. (An "open vent" at Stromboli allows magma to emerge almost continuously, with little explosions every 20 minutes or so.) At least 14 other volcanoes have been erupting for more than 30 years.

If you measure an eruption by how much magma gets expelled, the biggest ones aren't necessarily the longest-lasting. In 1912, the Novarupta volcano in Alaska released 15 cubic kilometers of material over just two and a half days. It would take a marathoner like Kilauea another two centuries of continuous eruption to match that total.

It may be possible to predict the length of an eruption once it starts. Volcanologists have identified a connection between the amount of energy released at the peak of an eruption and the average energy that's released over the whole course of the event. That means you can make a pretty good guess about how long the eruption will last if you can figure out when it's peaking.